Tunable triband graphene metasurface absorber with wide frequency agility for 6G and beyond communication in THz gap regime

A tunable triband metasurface absorber (TTMSA) based on interconnected octagonal concentric graphene rings (OCGRs) has been proposed. The effective capacitance between the OCGRs has been optimized to achieve near-unity absorptivity ($A\left(f\right)$) at 2.89, 4.38, and 6.33 THz by adjusting the position of the middle graphene ring. The simplicity of unit cell design enables the possible fabrication of TTMSAs through semiconductor fabrication technology. The symmetricity in the design leads to the polarization-insensitivity. Meanwhile, TTMSA shows angular stability for incidence angle ($\theta$) $\le$ 60° with $A\left(f\right)$ $\ge$ 90% under both transverse electric and magnetic polarizations. The TTMSA provides excellent frequency tunability at all three bands. The variation in ${\mu }_c$ from 0.3 to 1 eV leads to the frequency sweep from 2.23 to 8.9 THz, corresponding to 119.86% fractional bandwidth. The developed circuit model follows the results obtained from the simulations. The TTMSA is highly compact and ultra-thin, having periodicity and thickness of ${\lambda }_0$/12.98 and ${\lambda }_0$/51.91, respectively, making it suitable for size-constrained THz applications. The excitation of multiple plasmonic modes based on the surface plasmon polariton (SPP) resonance makes TTMSA a THz plasmonic device.